This dissertation is a study on the typical design of the emergency supply system in Hong Kong. The aim is to establish necessary considerations for the design of the emergency supply system for a typical building in compliance of the Fire Services Regulations and other codes of Practices. Further it investigates the selection of proper devices for ensuring a reliable emergency supply and protective device operation. Through case studies on several actual projects, it evaluates the probable defects within typical system designs and suggests improvements for designing similar systems in future. The dissertation firstly considers the electrical system design for a typical building and provides an investigation of their details found in the emergency supply system. By referring to the current Fire Services Regulations and the related Practices, the associated fire services installations for a typical building are studied. Also a description of the emergency supply system operation during a fire is included. By examining the standard specifications and operating characteristics of the typical components used in an emergency supply system, a proper understanding on their operation is investigated and an approach to their selection is established. Mathematical computations are carried out to evaluate the short-circuit conditions which can exist in the supply network and to predict their impact on system operation. Outcome is that certain guidelines and suggestion are formulated for improving the electrical system design for emergency supply systems.